Preclinical Imaging Evaluation of Novel TSPO-PET Ligand 2-(5,7-Diethyl-2-(4-(2-[18F]fluoroethoxy)phenyl)pyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide ([18F]VUIIS1008) in Glioma

Purpose

Translocator protein (TSPO) concentrations are elevated in glioma, suggesting a role for TSPO positron emission tomography (PET) imaging in this setting. In preclinical PET studies, we evaluated a novel, high-affinity TSPO PET ligand, [¹⁸F]VUIIS1008, in healthy mice and glioma-bearing rats.

Procedures

Dynamic PET data were acquired simultaneously with [¹⁸F]VUIIS1008 injection, with binding reversibility and specificity evaluated in vivo by non-radioactive ligand displacement or blocking. Compartmental analysis of PET data was performed using metabolite-corrected arterial input functions. Imaging was validated with histology and immunohistochemistry.

Results

[¹⁸F]VUIIS1008 exhibited rapid uptake in TSPO-rich organs. PET ligand uptake was displaceable with non-radioactive VUIIS1008 or PBR06 in mice. Tumor accumulation of [¹⁸F]VUIIS1008 was blocked by pretreatment with VUIIS1008 in rats. [¹⁸F]VUIIS1008 exhibited improved tumor-to-background ratio and higher binding potential in tumors compared to a structurally similar pyrazolopyrimidine TSPO ligand, [¹⁸F]DPA-714.

Conclusions

The PET ligand [¹⁸F]VUIIS1008 exhibits promising characteristics as a tracer for imaging glioma. Further translational studies appear warranted.     

Evaluation of 2-Deoxy-2-[18F]Fluoro-D-glucose- and 3′-Deoxy-3′-[18F]Fluorothymidine–Positron Emission Tomography as Biomarkers of Therapy Response in Platinum-Resistant Ovarian Cancer

Purpose

We evaluated whether 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) and 3??-deoxy-3??-[¹⁸F]fluorothymidine ([¹⁸F]FLT) positron emission tomography (PET) could be used as imaging biomarkers of platinum resensitization in ovarian cancer.

Procedures

Paired platinum-sensitive and platinum-resistant ovarian cancer cells from the same patient, PEO1 and PEO4, grown as tumor xenografts in nude mice, were assessed by PET.

Results

The AKT inhibitor, API-2, resensitized platinum-resistant PEO4 tumors to cisplatin, leading to a markedly lower Ki67 labeling index (p????0.006, n?=?6 per group). [¹⁸F]FDG-PET and [¹⁸F]FLT-PET imaging variables were lower after combination treatment compared with vehicle treatment (p????0.006, n?=?6 per group). No changes were seen with either drug alone. PRAS40 phosphorylation status was a sensitive biochemical marker of pathway inhibition, whereas reductions thymidine kinase 1 expression defined the [¹⁸F]FLT response.

Conclusions

Therapeutic inhibition of AKT activation in acquired platinum-resistant disease can be imaged noninvasively by [¹⁸F]FDG-PET and [¹⁸F]FLT-PET warranting further assessment.     

Synthesis and Ex Vivo Autoradiographic Evaluation of Ethyl-��-d-galactopyranosyl-(1,4��)-2��-deoxy-2��-[18F]fluoro-��-d-glucopyranoside��A Novel Radioligand for Lactose-Binding Protein: Implications for Early Detection of Pancreatic Carcinomas with PET

Introduction

Previous studies demonstrated that the lactose-binding protein (hepatocellular carcinoma?Cintestine?Cpancreas and pancreatitis-associated proteins (HIP/PAP)) is upregulated >130 times in peritumoral pancreatic tissue as compared to normal pancreatic tissue. Therefore, we developed a new radiolabeled ligand of HIP/PAP, the ethyl-??-d-galactopyranosyl-(1,4??)-2??-deoxy-2??-[¹⁸F]fluoro-??-d-glucopyranoside (Et-[¹⁸F]FDL) for noninvasive imaging of pancreatic carcinoma using positron emission tomography and computerized tomography (PET/CT).

Methods

The novel precursor and radiolabeling methods for synthesis of Et-[¹⁸F]FDL produced no isomers; the average decay-corrected radiochemical yield was 68%, radiochemical purity >99%, and specific activity >74 GBq/µmol. The radioligand properties of Et-[¹⁸F]FDL were evaluated using an ex vivo autoradiography and immunohistochemistry in pancreatic tissue sections obtained from mice-bearing orthotopic pancreatic tumor xenografts.

Results and Discussion

Et-[¹⁸F]FDL binding to peritumoral pancreatic tissue sections strongly correlated with HIP/PAP expression (r?=?0.81) and could be completely blocked by treatment with 1 mM lactose.

Conclusion

These results suggest that Et-[¹⁸F]FDL is a promising agent which should be evaluated for detection of early pancreatic carcinomas by PET/CT imaging.     

Improved PET Imaging of Tumors in Mice Using a Novel 18 F-Folate Conjugate with an Albumin-Binding Entity

Purpose

The folate receptor (FR) is a promising target for nuclear imaging due to its overexpression in many different cancer types. A drawback of using folate radioconjugates is the high accumulation of radioactivity in the kidneys. Therefore, the aim of this study was to develop a ¹⁸?F-labeled folate conjugate with an albumin-binding entity to enhance the blood circulation time and hence improve the tumor-to-kidney ratio.

Procedures

The novel ¹⁸?F-folate was prepared by conjugation of a ¹⁸?F-labeled glucose azide to an alkyne-functionalized folate precursor containing an albumin-binding entity via Cu(I)-catalyzed 1,3-dipolar cycloaddition. The radioconjugate was tested in vitro on FR-positive KB tumor cells and by biodistribution and positron emission tomography (PET) imaging studies using KB tumor-bearing mice.

Results

The radiosynthesis of the albumin-binding [¹⁸?F]fluorodeoxyglucose–folate ([¹⁸?F]3) resulted in a radiochemical yield of 1–2 % decay corrected (d.c.) and a radiochemical purity of ≥95 %. The specific activity of [¹⁸?F]3 ranged from 20 to 50 GBq/μmol. In vitro experiments revealed FR-specific binding of [¹⁸?F]3 to KB tumor cells. In vivo we found an increasing uptake of [¹⁸?F]3 into tumor xenografts over time reaching a value of ～?15 % injected dose (ID)/g at 4 h post-injection (p.i.). Uptake in the kidneys (～?13 % ID/g; 1 h p.i.) was approximately fourfold reduced compared to previously published ¹⁸?F-labeled folic acid derivatives. An excellent visualization of tumor xenografts with an unprecedentedly high tumor-to-kidney ratio (～?1) was obtained by PET imaging.

Conclusions

[¹⁸?F]3 showed a favorable accumulation in tumor xenografts compared to the same folate conjugate without albumin-binding properties. Moreover, the increased tumor-to-kidney ratios improved the PET imaging quality significantly, in spite of a somewhat higher background radioactivity which was a consequence of the slower blood clearance of [¹⁸?F]3.     

Increase of [18F]FLT Tumor Uptake In Vivo Mediated by FdUrd: Toward Improving Cell Proliferation Positron Emission Tomography

Purpose

3??-deoxy-3??-[¹⁸F]fluorothymidine ([¹⁸F]FLT), a cell proliferation positron emission tomography (PET) tracer, has been shown in numerous tumors to be more specific than 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) but less sensitive. We studied the capacity of a nontoxic concentration of 5-fluoro-2??-deoxyuridine (FdUrd), a thymidine synthesis inhibitor, to increase uptake of [¹⁸F]FLT in tumor xenografts.

Methods

The duration of the FdUrd effect in vivo on tumor cell cycling and thymidine analogue uptake was studied by varying FdUrd pretreatment timing and holding constant the timing of subsequent flow cytometry and 5-[¹²⁵I]iodo-2??-deoxyuridine biodistribution measurements. In [¹⁸F]FLT studies, FdUrd pretreatment was generally performed 1 h before radiotracer injection. [¹⁸F]FLT biodistributions were measured 1 to 3 h after radiotracer injection of mice grafted with five different human tumors and pretreated or not with FdUrd and compared with [¹⁸F]FDG tumor uptake. Using microPET, the dynamic distribution of [¹⁸F]FLT was followed for 1.5 h in FdUrd pretreated mice. High-field T2-weighted magnetic resonance imaging (MRI) and histology were used comparatively in assessing tumor viability and proliferation.

Results

FdUrd induced an immediate increase in tumor uptake of 5-[¹²⁵I]iodo-2??-deoxyuridine, that vanished after 6 h, as also confirmed by flow cytometry. Biodistribution measurements showed that FdUrd pretreatment increased [¹⁸F]FLT uptake in all tumors by factors of 3.2 to 7.8 compared with controls, while [¹⁸F]FDG tumor uptake was about fourfold and sixfold lower in breast cancers and lymphoma. Dynamic PET in FdUrd pretreated mice showed that [¹⁸F]FLT uptake in all tumors increased steadily up to 1.5 h. MRI showed a well-vascularized homogenous lymphoma with high [¹⁸F]FLT uptake, while in breast cancer, a central necrosis shown by MRI was inactive in PET, consistent with the histomorphological analysis.

Conclusion

We showed a reliable and significant uptake increase of [¹⁸F]FLT in different tumor xenografts after low-dose FdUrd pretreatment. These results show promise for a clinical application of FdUrd aimed at increasing the sensitivity of [¹⁸F]FLT PET.     

Improved Radiosynthesis and Biological Evaluations of L- and D-1-[<Superscript>18</Superscript>F]Fluoroethyl-Tryptophan for PET Imaging of IDO-Mediated Kynurenine Pathway of Tryptophan Metabolism

Purpose

Tryptophan metabolism via indoleamine 2,3-dioxygenase (IDO)-mediated kynurenine pathway plays a role in immunomodulation and has been emerging as a plausible target for cancer immunotherapy. Imaging IDO-mediated kynurenine pathway of tryptophan metabolism with positron emission tomography (PET) could provide valuable information for noninvasive assessment of cancer immunotherapy response. In this work, radiotracer 1-(2-[¹⁸F]fluoroethyl)-L-tryptophan (1-L-[¹⁸F]FETrp) and its enantioisomer 1-D-[¹⁸F]FETrp were synthesized and evaluated for PET imaging of IDO-mediated kynurenine pathway of tryptophan metabolism.

Procedures

Enantiopure 1-L-[¹⁸F]FETrp and 1-D-[¹⁸F]FETrp were prepared by a nucleophilic reaction of N-boc-1-(2-tosylethyl) tryptophan tert-butyl ester with [¹⁸F]Fluoride, followed by acid hydrolysis in a GE Tracerlab FX-N module. In vitro cell uptake assays were performed with a breast cancer cell line MDA-MB-231. Small animal PET/computed tomography (CT) imaging was carried out in a mouse model bearing MDA-MB-231 xenografts.

Results

Automatic radiosynthesis of 1-L-[¹⁸F]FETrp and 1-D-[¹⁸F]FETrp was achieved by a one-pot two-step approach in 19.0 ± 7.0 and 9.0 ± 3.0 % (n = 3) decay-corrected yield with radiochemical purity over 99 %, respectively. In vitro cell uptake study indicated the uptake of 1-D-[¹⁸F]FETrp in MDA-MB-231 cells was 0.73 ± 0.07 %/mg of protein at 60 min, while, the corresponding uptake of 1-L-[¹⁸F]FETrp was 6.60 ± 0.77 %/mg. Further mechanistic assays revealed that amino acid transport systems L-tpye amino acid transporter (LAT) and alanine-, serine-, and cysteine-preferring (ASC), and enzyme IDO expression were involved in cell uptake of 1-L-[¹⁸F]FETrp. Small animal PET/CT imaging study showed the tumor uptake of 1-L-[¹⁸F]FETrp was 4.6 ± 0.4 % ID/g, while, the tumor uptake of 1-D-[¹⁸F]FETrp was low to 1.0 ± 0.2 % ID/g, which were confirmed by ex vivo biodistribution study.

Conclusions

We have developed a practical method for the automatic radiosynthesis of 1-L-[¹⁸F]FETrp and 1-D-[¹⁸F]FETrp. Our biological evaluation results suggest that 1-L-[¹⁸F]FETrp is a promising radiotracer for PET imaging of IDO-mediated kynurenine pathway of tryptophan metabolism in cancer.

     

Evaluation of [18F]-CP18 as a PET Imaging Tracer for Apoptosis

Purpose

We identified and validated [¹⁸F]-CP18, a DEVD (the caspase 3 substrate recognition motif) containing substrate-based compound as an imaging tracer for caspase-3 activity in apoptotic cells.

Procedures

CP18 was radiolabeled with fluorine-18 using click chemistry. The affinity and selectivity of CP18 for caspase-3 were evaluated in vitro. The biodistribution and metabolism pattern of [¹⁸F]-CP18 were assessed in vivo. [¹⁸F]-CP18 positron emission tomography (PET) scans were performed in a dexamethasone-induced thymic apoptosis mouse model. After imaging, the mice were sacrificed, and individual organs were collected, measured in a gamma counter, and tested for caspase-3 activity.

Results

In vitro enzymatic caspase-3 assay demonstrated specific cleavage of CP18. In vivo, [¹⁸F]-CP18 is predominantly cleared through the kidneys and urine, and is rapidly eliminated from the bloodstream. There was a sixfold increase in caspase activity and a fourfold increase of [¹⁸F]-CP18 retention in the dexamethasone-induced thymus of treated versus control mice.

Conclusions

We report the use [¹⁸F]-CP18 as a PET tracer for imaging apoptosis. Our data support further development of this tracer for clinical PET applications.     

Evaluation of Arginine Deiminase Treatment in Melanoma Xenografts Using 18F-FLT PET

Purpose

This study aims to develop a molecular imaging strategy for response assessment of arginine deiminase (ADI) treatment in melanoma xenografts using 3′-[¹⁸F]fluoro-3′-deoxythymidine ([¹⁸F]-FLT) positron emission tomography (PET).

Procedures

F-FLT response to ADI therapy was studied in preclinical models of melanoma in vitro and in vivo. The molecular mechanism of response to ADI therapy was investigated, with a particular emphasis on biological pathways known to regulate ¹⁸F-FLT metabolism.

Results

Proliferation of SK-MEL-28 melanoma tumors was potently inhibited by ADI treatment. However, no metabolic response was observed in FLT PET, presumably based on the known ADI-induced degradation of PTEN, followed by instability of the tumor suppressor p53 and a relative overexpression of thymidine kinase 1, the enzyme mainly responsible for intracellular FLT processing.

Conclusion

The specific pharmacological properties of ADI preclude using ¹⁸F-FLT to evaluate clinical response in melanoma and argue for further studies to explore the use of other clinically applicable PET tracers in ADI treatment.     

Comparison of 18F-labeled CXCR4 antagonist peptides for PET imaging of CXCR4 expression

Purpose

CXCR4 is overexpressed on tumor cells from many types of human cancers. A high level of CXCR4 expression often correlates with poor prognosis, chemotherapy resistance, and metastasis. The development of CXCR4-specific radiotracers for positron emission tomography (PET) imaging will allow in vivo evaluation of receptor expression level for diagnosis or therapeutic evaluation.

Procedures

Two new ¹⁸F-labeled radiotracers based on an Ac-TC14012 peptide, [¹⁸F]FP-Ac-TC14012 and [¹⁸F]FB-Ac-TC14012, were synthesized and characterized. The affinities of the 2-fluoropropionate (FP)-conjugated or 4-fluorobenzoate (FB)-conjugated peptides to CXCR4-transfected Chinese hamster ovarian (CHO) cells were evaluated in a competitive binding assay with [¹²⁵I]CXCL12 radioligand. The cell uptake and retention of [¹⁸F]FP-labeled and [¹⁸F]FB-labeled peptides were measured. The tumor targetability and pharmacokinetics of these two tracers were also evaluated by microPET imaging and biodistribution studies.

Results

The labeled peptides retained high binding affinity to CXCR4 and showed much higher uptake in CXCR4-positive CHO cells than in CXCR4-negative cells in vitro. The smaller and more hydrophilic [¹⁸F]FP prosthetic group resulted in higher affinity and lower nonspecific cell uptake compared to the [¹⁸F]FB-labeled peptide. Both radiotracers showed much higher accumulation in CXCR4-positive than CXCR4-negative tumor xenografts in mice and allowed clear visualization of CXCR4 expression by PET. Among the two, [¹⁸F]FP-Ac-TC14012 showed higher tumor uptake and better tumor-to-background contrast. Unlike their N-terminal 4-F-benzoate analogs, these two tracers had minimal blood retention, likely due to reduced red blood cell binding. Metabolic organs, such as the liver and kidney, also showed high uptake. When blocked with low-dose cold peptide (10 μg), the tumor uptake was significantly increased, most likely due to the increased concentration in blood circulation, as evidenced by decreased liver uptake.

Conclusion

These results demonstrate that the [¹⁸F]FP-labeled Ac-TC14012 peptide with high tumor uptake, low nonspecific binding, and good tumor-to-background contrast promises [¹⁸F]FP-Ac-TC14012 as a PET tracer for in vivo PET imaging of CXCR4 expression.     

Examining Changes in [18 F]FDG and [18 F]FLT Uptake in U87-MG Glioma Xenografts as Early Response Biomarkers to Treatment with the Dual mTOR1/2 Inhibitor AZD8055

Purpose

The mTOR kinase inhibitor AZD8055 inhibits both mTORC1 and mTORC2 leading to disruption of glucose metabolism and proliferation pathways. This study assessed the impact of single and multiple doses of AZD8055 on the uptake of the glucose metabolism marker 2-deoxy-2-[¹⁸?F]fluoro-d-glucose ([¹⁸?F]FDG) and the proliferation marker 3′-deoxy-3′-[¹⁸?F]fluorothymidine ([¹⁸?F]FLT) in U87-MG glioma xenografts.

Procedures

Mice bearing U87-MG tumours received either vehicle or AZD8055 (20 mg/kg) once daily p.o. Mice were imaged with either [¹⁸?F]FDG or [¹⁸?F]FLT PET to assess treatment response. Comparisons were made between in vivo imaging and ex vivo histopathology data.

Results

Tumour uptake of [¹⁸?F]FDG was reduced by 33 % 1 h after a single dose of AZD8055 and by 49 % following 4 days of dosing. These changes coincided with suppression of the mTOR pathway biomarkers pS6 and pAKT. In contrast, the effect of AZD8055 on [¹⁸?F]FLT uptake was inconsistent.

Conclusions

The very rapid change in [¹⁸?F]FDG uptake following acute AZD8055 treatment suggests that this could be used as an early mechanistic biomarker of metabolic changes resulting from mTOR inhibition. The utility of [¹⁸?F]FLT for measuring the anti-proliferative effect of AZD8055 remains unclear.     

PET Imaging of Stroke-Induced Neuroinflammation in Mice Using [18F]PBR06

Purpose

The purpose of this study is to evaluate the 18 kDa translocator protein (TSPO) radioligand [¹⁸F]N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline ([¹⁸F]PBR06) as a positron emission tomography (PET) imaging biomarker of stroke-induced neuroinflammation in a rodent model.

Procedures

Stroke was induced by transient middle cerebral artery occlusion in Balb/c mice. Dynamic PET/CT imaging with displacement and preblocking using PK111195 was performed 3 days later. PET data were correlated with immunohistochemistry (IHC) for the activated microglial markers TSPO and CD68 and with autoradiography.

Results

[¹⁸F]PBR06 accumulation peaked within the first 5 min postinjection, then decreased gradually, remaining significantly higher in infarct compared to noninfarct regions. Displacement or preblocking with PK11195 eliminated the difference in [¹⁸F]PBR06 uptake between infarct and noninfarct regions. Autoradiography and IHC correlated well spatially with uptake on PET.

Conclusions

[¹⁸F]PBR06 PET specifically images TSPO in microglial neuroinflammation in a mouse model of stroke and shows promise for imaging and monitoring microglial activation/neuroinflammation in other disease models.     

2-[18F]Fludarabine, a Novel Positron Emission Tomography (PET) Tracer for Imaging Lymphoma: a Micro-PET Study in Murine Models

Purpose

Fludarabine has proven to be of considerable efficacy in the treatment of low-grade lymphomas. We have developed the labeling of this drug with fluorine-18 and evaluated 2-[¹⁸F]fludarabine as a novel positron emission tomography (PET) probe for in vivo imaging.

Procedures

Preclinical studies were conducted with 2-[¹⁸F]fludarabine, in parallel with 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG), in Swiss CD-1 and CB17 severely combined immunodeficient (SCID) mice, both as tumor-free control groups, and SCID mice bearing RL lymphomas.

Results

In Swiss mice, micro-PET studies with 2-[¹⁸F]fludarabine showed a distribution restricted to the organs of excretion and the spleen, the latter being less evident in SCID animals. In lymphoma-bearing SCID mice, 2-[¹⁸F]fludarabine demonstrated a rapid tumor uptake over the first 20 min which subsequently plateaued and provided an improved contrast than that of [¹⁸F]FDG.

Conclusion

This radiotracer merits further evaluation to establish its clinical usefulness to image low-grade lymphoma in humans in future clinical investigations.     

6-[18F]Fluoro-l-DOPA Uptake in the Rat Pancreas is Dependent on the Tracer Metabolism

Purpose

6-[¹⁸F]fluoro-l-3,4-dihydroxyphenyl alanine ([¹⁸F]FDOPA) positron emission tomography (PET) is a diagnostic tool which can detect malignancies of the pancreas. We aimed to study whether the manipulation of the [¹⁸F]FDOPA metabolic pathway would change the ¹⁸F-behavior to provide a biochemical foundation for PET imaging of rat pancreas with [¹⁸F]FDOPA.

Procedures

Inhibitors of aromatic amino acid decarboxylase, catechol-O-methyltransferase, monoamine oxidases A and B, or their combinations on [¹⁸F]FDOPA uptake, metabolism, and the regional distribution in the rat pancreas was evaluated using in vivo PET/computed tomography imaging, chromatographic metabolite analyses, and autoradiography.

Results

Enzyme inhibition generally increased the uptake of [¹⁸F]FDOPA derived ¹⁸F-radioactivity in rat pancreas. Dependent on which enzymatic pathway is blocked (or a combination of pathways), different radiolabeled metabolites in pancreas are responsible for this increase in uptake.

Conclusions

Altering the metabolism of [¹⁸F]FDOPA by using various enzymatic inhibitors increased the radioactivity uptake and changed the radiometabolic profile in the pancreas allowing better discrimination between pancreas and surrounding tissues of rat. However, these manipulations did not separate islets from the exocrine pancreas. Elucidating the metabolic behavior of [¹⁸F]FDOPA provides a biochemical foundation of PET imaging of the rat pancreas.     

In Vitro and In Vivo Evaluation of the Caspase-3 Substrate-Based Radiotracer [18F]-CP18 for PET Imaging of Apoptosis in Tumors

Purpose

A novel caspase-3 substrate-based probe [¹⁸F]-CP18 was evaluated as an in vivo positron emission tomography (PET) imaging agent for monitoring apoptosis in tumors.

Methods

Uptake of [¹⁸F]-CP18 in cell assays and tumors was measured. Caspase-3/7 activities in cell lysates and tumor homogenates were determined. Autoradiography,Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and cleaved caspase-3 immunostaining were performed on adjacent tumor sections to identify areas of apoptosis.

Results

The in vitro cell assays showed caspase-3-dependent uptake of [¹⁸F]-CP18 in tumor cells when treated with an apoptosis inducer. The in vivo microPET imaging signal of [¹⁸F]-CP18 in xenograft tumors correlated with the ex vivo caspase-3/7 activities in these tumors. Furthermore, tumor autoradiographies of [¹⁸F]-CP18 in tumor sections matched adjacent sections stained by TUNEL and caspase-3 immunohistochemistry (IHC).

Conclusions

[¹⁸F]-CP18 demonstrated high affinity and selectivity for activated caspase-3 both in vitro and in vivo, and the results support [¹⁸F]-CP18 as a promising new PET imaging agent for apoptosis.     

PET Imaging on Dynamic Metabolic Changes after Combination Therapy of Paclitaxel and the Traditional Chinese Medicine in Breast Cancer-Bearing Mice

Purpose

The aim of the study was to non-invasively evaluate the anticancer activity of a traditional Chinese medicine—Huaier, combined with paclitaxel (PTX) in breast cancer bearing mice by detecting dynamic metabolic changes with positron emission tomography (PET).

Procedures

Balb/c nude mice were randomly divided into one of the four groups: Huaier, PTX, PTX + Huaier, or the control. PET imaging with 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) was performed to monitor the metabolic changes in BT474 (luminal B) and MDA-MB-231 (triple-negative) breast cancer xenografts. Immunohistochemistry (IHC) study was performed immediately after the final PET scan to assess the expressions of phosphatidylinositol 3-kinase (PI3K), phospho-AKT (p-AKT), caspase-3, and vascular endothelial growth factor (VEGF).

Results

Compared to the control group, [¹⁸F]FDG accumulation demonstrated a significant decrease in PTX + Huaier (p < 0.01) or Huaier group (p < 0.05), which was consistent to the decreased expression of PI3K (p < 0.05) and p-AKT (p < 0.05) in the breast cancer xenografts.

Conclusion

The therapeutic effect of Huaier combined with PTX was superior than the PTX alone in BT474 and MDA-MB-231 breast cancer-bearing mice. [¹⁸F]FDG PET imaging could be a potential non-invasive approach to assess the metabolic changes after chemotherapy combined with traditional Chinese medicine in the breast cancer.

     

A 18F-Labeled BF-227 Derivative as a Potential Radioligand for Imaging Dense Amyloid Plaques by Positron Emission Tomography

Purpose

The aims of this study were to evaluate the binding and pharmacokinetics of novel ¹⁸F-labeled ethenyl-benzoxazole derivatives (i.e., [¹⁸F] fluorinated amyloid imaging compound of Tohoku university ([¹⁸F]FACT)) as amyloid positron emission tomography (PET) tracers and to assess [¹⁸F]FACT efficacy in imaging of Alzheimer’s disease (AD).

Procedures

Binding assay was conducted using synthetic amyloid-β (Aβ) fibrils, fluorescence microscopy, and autoradiogram in three postmortem AD brains. Pharmacokinetics of [¹⁸F]FACT was assessed using 12 Crj:CD-1 (ICR) mice. In vivo binding ability with brain amyloid was investigated using amyloid precursor protein (APP) transgenic mouse. Clinical PET scanning using [¹⁸F]FACT was performed in ten healthy controls and ten mild cognitive impairment and ten AD patients.

Results

[¹⁸F]FACT showed high binding affinity for synthetic Aβ fibrils, preferential binding to dense cored plaques in brain sections, and excellent brain uptake and rapid clearance in mice. Injection in APP mice resulted in specific in vivo labeling of amyloid deposits in the brain. PET scans of AD patients showed significantly higher [¹⁸F]FACT uptake in the neocortex compared to controls (P?<?0.05, Kruskal–Wallis test).

Conclusion

[¹⁸F]FACT is a promising agent for imaging dense Aβ plaques in AD.     

Electrophilic Addition of Chlorine Monofluoride for PET tracers

Purpose

We have studied the utility of [¹⁸F]ClF electrophilic addition to the carbon–carbon double bond of analogues of a model positron emission tomography (PET) tracer, [¹⁸F]EF5. The consequence of simultaneous chlorine/fluorine addition on lipophilicity and biological activity of the molecule is evaluated.

Procedures

Post-target produced [¹⁸F]F₂ was reacted with Cl₂ to produce [¹⁸F]ClF, which was used in electrophilic addition.

Results

[¹⁸F]ClF was produced and used to label chlorinated analogues of [¹⁸F]EF5. The chlorinated analogues, [¹⁸F]EF4Cl_a and [¹⁸F]EF4Cl_b, were synthesized simultaneously. The in vivo uptake of the analogues compared well with [¹⁸F]EF5 uptake in tumor-bearing mice.

Conclusion

[¹⁸F]ClF is a suitable labeling reagent for electrophilic addition to double bonds of PET tracers. The results show that the modification of the pentafluoro group of [¹⁸F]EF5 by monofluorine-for-chlorine exchange affected the lipophilicity, but the hypoxia avidity of these molecules was not apparently altered.     

Differences in Transport Mechanisms of trans-1-Amino-3-[18F]Fluorocyclobutanecarboxylic Acid in Inflammation,Prostate Cancer,and Glioma Cells: Comparison with l-[Methyl-11C]Methionine and 2-Deoxy-2-[18F]Fluoro-d-Glucose

Purpose

We aimed to elucidate trans-1-amino-3-[¹⁸F]fluorocyclobutanecarboxylic acid (anti-[¹⁸F]FACBC) uptake mechanisms in inflammatory and tumor cells, in comparison with those of l-[methyl-¹¹C]methionine ([¹¹C]Met) and 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG).

Procedures

Using carbon-14-labeled tracers, in vitro time-course, pH dependence, and competitive inhibition uptake experiments were performed in rat inflammatory (T cells, B cells, granulocytes, macrophages), prostate cancer (MLLB2), and glioma (C6) cells.

Results

Anti-[¹⁴C]FACBC uptake ratios of T/B cells to tumor cells were comparable, while those of granulocytes/macrophages to tumor cells were lower than those for [¹⁴C]FDG. Over half of anti-[¹⁴C]FACBC uptake by T/B and tumor cells was mediated by Na⁺-dependent amino acid transporters (system ASC), whereas most [¹⁴C]Met transport in all cells was mediated by Na⁺-independent carriers (system L).

Conclusions

The low anti-[¹⁸F]FACBC accumulation in granulocytes/macrophages may be advantageous in discriminating inflamed regions from tumors. The significant anti-[¹⁸F]FACBC uptake in T/B cells may cause false-positives in some cancer patients who undergo FACBC-positron emission tomography (PET).     

Suitability of [18F]Altanserin and PET to Determine 5-HT2A Receptor Availability in the Rat Brain: In Vivo and In Vitro Validation of Invasive and Non-Invasive Kinetic Models

Purpose

While the selective 5-hydroxytryptamine type 2a receptor (5-HT_2AR) radiotracer [¹⁸F]altanserin is well established in humans, the present study evaluated its suitability for quantifying cerebral 5-HT_2ARs with positron emission tomography (PET) in albino rats.

Procedures

Ten Sprague Dawley rats underwent 180 min PET scans with arterial blood sampling. Reference tissue methods were evaluated on the basis of invasive kinetic models with metabolite-corrected arterial input functions. In vivo 5-HT_2AR quantification with PET was validated by in vitro autoradiographic saturation experiments in the same animals.

Result

Overall brain uptake of [¹⁸F]altanserin was reliably quantified by invasive and non-invasive models with the cerebellum as reference region shown by linear correlation of outcome parameters. Unlike in humans, no lipophilic metabolites occurred so that brain activity derived solely from parent compound. PET data correlated very well with in vitro autoradiographic data of the same animals.

Conclusion

[¹⁸F]Altanserin PET is a reliable tool for in vivo quantification of 5-HT_2AR availability in albino rats. Models based on both blood input and reference tissue describe radiotracer kinetics adequately. Low cerebral tracer uptake might, however, cause restrictions in experimental usage.     

Fusion of [18F]FDG PET with Fluorescence Diffuse Optical Tomography to Improve Validation of Probes and Tumor Imaging

Purpose

Given the progress of fluorescence diffuse optical tomography (fDOT) technology, here, we study the additional benefits provided by multimodal PET/fDOT imaging by comparing the biodistribution of 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) in tumors with three fluorescent probes: a glucose analog, a protease activatable optical probe, and a ligand of αvβ3 integrin.

Procedures

Sequential fDOT/PET/computed tomography (CT) imaging of mice was performed with a custom multimodal mouse support that allows the subject to be transferred between the fDOT and the PET/CT scanners. Experiments were performed in xenografted tumor models derived from the human breast cancer line MDA-MB 231 and compared to ex vivo analysis.

Results

The three-dimensional signals showed that the fluorescent glucose analog is not colocalized with [¹⁸F]FDG, raising questions about its use as a surrogate probe of the PET tracer. Fusion of [¹⁸F]FDG with the other fluorescent probes showed evidence of high variability both for the protease activity and the αvβ3 integrin expression during tumor growth.

Conclusion

The added value of hybrid PET/fDOT over the two modalities was demonstrated for cross-validation of probes and for better characterization of tumor models.